The present invention relates to a developing roller and, more particularly, to a developing roller constituting a developing device of an electrophotographic copier, facsimile apparatus, printer or similar image recorder and held in contact with an image carrier of the image recorder for developing a latent image in the form of an electrostatic charge deposited on the image carrier by use of a dry one-component toner.
Dry process development adopted in the art of electrophotography and electrostatic recording which include an electrophotographic copier, facsimile apparatus and laser printer is implemented by either a two-component developer on a one-component developer. While a two-component developer is made up of toner and carrier, a one-component developer is constituted by toner only, as well known in the art. Image development using a two-component developer stably produces images of high quality, but cannot reproduce such images over a long period of time due to problems such as deterioration of the carrier and varying mixture ratio of the toner and carrier, etc. Another problem with the two-component developer is that a recording apparatus using such a two component carrier cannot be maintained or provided in a compact configuration with ease. In the light of this, image development using a one-component developer, which is free from such problems, is attracting increasing attention. The one-component developer, i.e., toner, may either be a magnetic toner or a non-magnetic toner.
In a developing system using a one-component toner as the developer, toner is supplied to a developing roller to develop a latent image in the form of an electrostatic charge which is formed on a photoconductive drum or similar image carrier. Among the many prerequisites for a developing roller of the type using non-magnetic toner, one is that it be provided with adequate elasticity and, preferably, with a predetermined conductivity. Typical of prior art developing rollers is a roller made up of a metal core, an elastic layer provided on the metal core, and a toner retaining layer provided on the elastic layer. However, the elastic layer has an electric resistivity of 10.sup.10 .OMEGA. cm to 10.sup.16 .OMEGA. cm, which is too high to cope with varying temperature and humidity. While conductive powder which serves as a conducting agent may be added to the elastic layer to lower its resistivity as heretofore practiced, such an approach brings about another problem in that elasticity is lost due to the substantial content of the conductive powder and, therefore, the developing roller fails to sufficiently make contact with the photoconductive drum during development. These related problems are usually dealt with by adding not only the conducting agent but also a substantial amount of softener to the elastic layer to assure effective contact of the developing roller and the drum. However, when this kind of prior art developing roller is used, various undesirable phenomena occur, such as the oooze-out of the softener through the surface of the toner retaining layer and the opposite ends of the elastic layer. The softener so oozing out causes the toner to harden.